Short answers, pulled from the story.
A group of armed terrorists from Chechnya and other regions attacked School Number One in Beslan. The attackers included 33 mujahideen such as Shamil Basayev who claimed responsibility for the operation.
The gymnasium roof collapsed at 13:05 on the third day of the siege after an explosion started a fire. This event killed some 160 people and filled the room with flames.
The attackers took approximately 1,100 hostages into the school building. More than 1,200 people were injured and over 437 including 221 children were hospitalized.
Conflicting reports exist regarding the source of the explosions that destroyed the gymnasium roof. Some witnesses claim federal forces fired rockets while others state militants detonated bombs among hostages.
Major Sultan Gurazhev alerted authorities after being captured by terrorists near Khurikau. He ran to the district police department to report the situation before the attackers reached the school.